Since discharge lights can provide high luminance irradiation, these discharge lights are frequently used not only for outdoor lighting and roadway lighting, but also for motor vehicle headlights and interior lighting for stores or the like. As a light source, an arc tube such as that shown in FIG. 1(6) is known.
That is, arc tube 2 has a glass tube 4 having a spherical portion 4a formed in the middle thereof, and a pair of electrode assembly unit 6 arranged on both sides of the spherical portion 4a within the glass tube 4. Each electrode assembly unit 6 has both an electrode rod 8 that projects into an inner space of the spherical portion 4a (discharge chamber) and a lead wire 10 that projects from an end portion of the glass tube 4 connected thereto through a rectangular molybdenum foil 12, and is pinch-sealed to the glass tube 4 at the molybdenum foil 12 portion. By selecting the structure in which each molybdenum foil 12 is interposed between the electrode rod 8 and the lead wire 10 and the molybdenum foil 12 portions are pinch-sealed to the glass tube 4, a difference between the thermal expansion of a metal electrode made of a single member and that of the glass tube 4 when such metal electrode made of a single member is used can be absorbed by the thin film molybdenum foils 12, so that sealability within the spherical portion 4a can be maintained.
The term "pinch-seal" as used herein is intended to mean a sealing method in which an object to be inserted (such as a molybdenum foil) into the glass tube is embedded into the glass tube, with the object brought into intimate contact with the glass tube element by compressing the glass tube with a press while heated.
The aforementioned pair of molybdenum foils 12 are sequentially pinch-sealed on a single foil basis. Such pinch-seal operation for the first molybdenum foil has heretofore been performed in the following manner. As shown in FIG. 6, the electrode assembly unit 6 is inserted from one end portion of the glass tube 4 so that the molybdenum foil 12 is positioned close to the spherical portion 4a of the glass tube 4 (condition (a)), and under this condition, not only an inert gas such as argon gas or nitrogen gas is introduced into the glass tube 4 to thereby drive the air out of the glass tube 4, but also the portion of the glass tube 4 surrounding the molybdenum foil 12 is heated (condition (b)), and then, the glass tube 4 is pressed with a pincher 22 (condition (c)) to provide a pinch-sealed entity (condition (d)). This is how the molybdenum foil is pinch-sealed.
However, in the aforementioned conventional pinch sealing method, the following problems arise.
Since the tensile strength of a molybdenum foil is reduced when the molybdenum foil is oxidized, the molybdenum foil is susceptible to breakage during a pinch sealing operation. As a result, in the conventional pinch sealing method, the air that causes oxidation of the molybdenum foil is driven out of the glass tube 4 by introducing an inert gas into the glass tube. However, this method is not effective to adequately remove the air within the glass tube 4. As a result, the oxidation preventing effect of such a method is not enough either, causing quite a few molybdenum foil breakages.
In particular, in order to allow the molybdenum foils to absorb the difference between the thermal expansion coefficient of the molybdenum foil and that of the glass tube sufficiently, it is preferred that the thickness of each molybdenum foil be as thin as possible. However, if the thickness of the molybdenum foil is reduced to as thin as 20 .mu.m or less, the problem of molybdenum breakage has often been encountered by the aforementioned conventional pinch sealing method.
On the other hand, if a molybdenum foil is formed by adding an additive such as potassium or calcium to molybdenum, the incidence of the aforementioned breakage can be reduced even if the thickness of the molybdenum foil is reduced to as thin as 20 .mu.m or less. However, when such molybdenum foil having an additive mixed is used, the molybdenum foil becomes expensive, and the molybdenum foil is hard to machine with the hardness thereof increased.